Ultrasonic rangefinder

Abstract

An ultrasonic rangefinder providing a correct distance measurement and comprising a first transducer for transducing electric signals into ultrasonic waves and transmitting the ultrasonic waves, a second transducer for receiving the ultrasonic waves and transducing the ultrasonic waves into electric signals, a differentiation circuit, and a comparator for comparing a level of a differentiated output signal from the differentiation circuit with an output signal level of the second transducer and detecting a threshold time when the level of the differentiated output signal from the differentiation circuit becomes lower then the output signal level of the second transducer where a threshold signal level of the comparator is automatically varied with respect to a time lapse of the travelling ultrasonic waves coming back to the rangefinder after being reflected by an object, thus overcoming the conventional problems due to the spurious-like direct waves.

What is claimed is: 
     
       1. An ultrasonic rangefinder comprising: a first transducer for transducing electric signals into ultrasonic waves and transmitting said ultrasonic waves,   a second transducer for receiving said ultrasonic waves and transducing said ultrasonic waves into electric signals,   a first timer means for determining a starting time for transmitting said ultrasonic waves,   a carrier frequency generator for generating a carrier signal of a frequency of said ultrasonic waves,   a second timer means for generating a pulse having a time width at least as long as a time corresponding to a travelling time of said ultrasonic waves from said first transducer to said second transducer at the time when said first transducer begins to transmit said ultrasonic waves,   a differentiation circuit for differentiating said pulse of said second timer means, and   a comparator for comparing a level of said differentiated output signal from said differentiation circuit with an output signal level of said second transducer and detecting a threshold time when said level of said differentiated output signal from said differentiation circuit means becomes lower than said output signal level of said second transducer.   
     
     
       2. An ultrasonic rangefinder in accordance with claim 1 wherein said carrier frequency generator comprises a compensation means for compensating changes of a sound velocity due to the temperature variations by changing said carrier signal frequency. 
     
     
       3. An ultrasonic rangefinder in accordance with claim 1 or 2 wherein said carrier signal frequency is a frequency around 34 KHz. 
     
     
       4. An ultrasonic rangefinder in accordance with claim 1 wherein said second timer means is for generating a pulse having a time width longer than a time corresponding to a travelling time of said ultrasonic waves from said first transducer to said second transducer at the time when said first transducer begins to transmit said ultrasonic waves. 
     
     
       5. An ultrasonic rangefinder comprising: a first transducer for transducing electric signals into ultrasonic waves and transmitting said ultrasonic waves,   a second transducer for receiving said ultrasonic waves and transducing said ultrasonic waves into electric signals,   a first timer means for determining a starting time for transmitting said ultrasonic waves,   a carrier frequency generator for generating a carrier signal of a frequency of said ultrasonic waves,   a second timer means for generating a pulse having a time width at least as long as a time corresponding to a travelling time of said ultrasonic waves from said first transducer to said second transducer at the time when said first transducer begins to transmit said ultrasonic waves,   a differentiation circuit for differentiating said pulse of said second timer means,   a comparator for comparing a level of said differentiated output signal from said differentiation circuit with an output signal level of said second transducer and detecting a threshold time when said level of said differentiated output signal from said differentiation circuit becomes lower than said output signal level of said second transducer,   a logic gate for passing a burst signal made from a pulse signal of said first timer means and said carrier signal of said carrier frequency generator,   a first amplifier for amplifying said burst signal, and   a second amplifier for amplifying said electric signals of said second transducer.   
     
     
       6. An ultrasonic rangefinder in accordance with claim 5 further comprises a flip-flop for being set at the time when said first transducer begins to transmit said ultrasonic waves and being then reset by an output signal of said comparator. 
     
     
       7. An ultrasonic rangefinder in accordance with claim 5 or 6 wherein said carrier frequency generator comprises a compensation means for compensating changes of a sound velocity due to the temperature variations by changing said carrier signal frequency. 
     
     
       8. An ultrasonic rangefinder in accordance with claim 5 or 6 wherein said carrier signal frequency is a frequency around 34 KHz. 
     
     
       9. An ultrasonic rangefinder in accordance with claim 7 wherein said second timer means is for generating a pulse having a time width longer than a time corresponding to a travelling time of said ultrasonic waves from said first transducer to said second transducer at the time when said first transducer begins to transmit said ultrasonic waves. 
     
     
       10. An ultrasonic rangefinder in accordance with claim 5 or 6 further comprising an indication means for indicating a distance value.